[Twisted-Python] twisted listening on UDP port, why?

[Jonathan Vanasco]

On Apr 20, 2016, at 1:31 PM, Jonathan Vanasco wrote:

> A specific library was keeping the port open.  I'm tracking down how/why right now.

So this was fun <sarcasm> thing to learn...

An undocumented (yay) feature of python appears to be... python binds to a random port on all interfaces (0.0.0.0) once you send UDP data through it.  I assume this is to allow for a response to come back.  

We're using statsd for metrics in our twisted daemon and detect issues post-deployment.

If you haven't used it, it's a node.js daemon from etsy that collects udp data and pipes it into python's graphite/carbon libraries.  Then you get fancy graphics.

    # this does nothing...
    sock = socket.socket(family, socket.SOCK_DGRAM)
    # but this binds to 0.0.0.0
    sock.sendto(data.encode('ascii'), addr)

Sending data to the stats collector on 127.0.0.1:8125 inherently made python bind to 0.0.0.0, and on a port that seems to be in the 40000-60000 range.  
Since a socket to the stats collector is only created once for the process, Python holds that open the entire time.